How to Read a Pump Curve

Lead In Photo Pump Curves

Pump Curves give you the information you need to choose the right pump for your application.

In this blog, we'll show you how to read a pump curve so you can base your pump selection on the specific needs of your application.

A basic pump curve provides two pieces of information about the pump's capacity to move water: head and flow.

1. Head is simply the height to which a pump can raise water straight up. Water creates pressure or resistance, at predictable rates, so we can calculate head as the differential pressure that a pump has to overcome in rder to raise the water. Common units are feet of head and pounds per square inch. As figure 1 illustrates, every 2.31 feet of head equals 1 PSI of pressure.

Pump - Feet of Head
Fig. 1. Every 2.31 feet of head creates 1 PSI of pressure.

2. Flow is the volume of water a pump can move at a given pressure. Flow is typically indicated on the horizontal axis in unites like gallons per minute, or gallons per hour, as shown in Figure 2.

S15 Pump Curve
Fig. 2. A basic pump curve shows a pump's performance range. In this curve, head is measured in PSI and flow is measured in gallons per hour.

Reading the sample curve

As an example, let's say you want to know the flow rate to expect from the pump in Figure 3 at 60 Hz when the design pressure is 80 PSI. Looked at another way, this pump can generate 80 PSI of discharge pressure at a flow rate of 1321 gallons per hour.

Reading a Pump Curve
Fig. 3. A pump that operates at a constant speed will perform according to the curve; pumps that operate at variable speeds can perform above or below the curve.

Reading curves that contain additional information

Because some pumps can operate at a range of horsepower, their curves will include additional information. Figure 4, for example, features a pump that can operate from 2 to 10 horsepower depending on desired performance.

Pump Curve with Additional Information
Fig. 4. Pump curves showing horsepower enable you to choose the right impeller and motor combination based on application demands.

In additional to standard PSI and flow, the curve at the bottom of Figure 4 indicates NPSHr, which stands for Net Positive Suction Head Required. NPSHr is the minimum amount of pressure required on the suction side of the pump to avoid cavitation, or the introduction of air into the fluid stream. NPSHr is determined by the pump.

The other side of the equation is NPSHa, with "a" standing for available. NPSHa is determined by the process piping. You always want NPSHa to be greater than NPSHr. Without enough net positive suction, the pump will capitate, which affects performance and pump life.

Finding maximum pump efficiency

In general, the middle of the curve is where a pump is operating at maximum efficiency in terms of pressure and flow rate.

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